Injection molding processes typically employ molds of a design whereby the molded part is formed in one or more cavities created by at least two mold halves mounted upon the respective stationary and movable platens of the molding machine. The mold cavity thus formed is opened or closed at the parting line junction of the two halves according to a predetermined control sequence of the molding machine and its press cycle.
Typically the press cycle includes a mold-open portion, during which the mold halves open along the plane of the parting line and the molded part or article is removed from the mold's part-forming surfaces. During this mold-open portion of the molding cycle, the molded part or parts are ejected off the mold surface and are alternately either allowed to fall free or, if the molded part is likely to be damaged by such random tumbling action, the part must be grabbed onto either manually (by a press-side operator) or by an automatic part-removal device operating in sequence with the opening and closing portions of the press cycle.
A stack mold is a special case of this conventional injection molding process, in which two parting-lines form respective mold cavities which are operated in coordinated sequence through well-known mechanical linkages (such as rack-and-pinion means). One mold half is mounted upon each of the two machine platens and closing upon a centrally-mounted moldstack having on each of its faces at least one corresponding mold cavity half. So a stack mold functions in the same manner as conventional molds, just with a plurality of opening and closing parting lines.
Certain "white room" injection molding requires greater control over molded-part surface cleanliness and flaws. Examples are dust lodging onto an optical lens molding cavity surface during its open cycle and thus forming a pit flaw on the resulting molded part, or concern over microbiological surface contamination and part cleanliness such as would be the case for food packaging and medical devices or prostheses, especially those implanted surgically into human bodies. Requirements for such "white room" molding revolve around rather expensive clean-air handling systems needed for Class 10,000 down to Class 10 airflows.
These classifications correspond to the purity of the air; a Class 10,000 room has 10,000 particles of greater than 0.5 micron diameter per cubic foot of air, the Class 10 only 10 per cubic foot.
In addition, complex part-removal mechanisms with programmable controls and sophisticated end-of-arm tooling are frequently required in order to prevent damage to the part (human operators create unacceptable disruption of clean-room laminar air flows and standard; if people have to remove the part manually from between the mold halves, real cleanliness is impossible). Such automated robotic systems commonly cost $30,000-60,000 per mold and molding machine.
Yet it is not cost alone which the current state-of-arts (see "Clean-room molding: an attractive specialty" article in PLASTICS MACHINERY & EQUIPMENT Magazine, February 1989, pp. 33-37.) in clean-room molding and part removal leaves room for improvement. A very fundamental problem is that the mold surfaces give off substantial quantities of heat on each molding cycle, causing convective air currents which creates a "chimney" effect. Warm air rises off the heated mold surfaces to convectively create a draft which sucks "dirtier" ambient-temperature air with airborne particles and contaminants in from the floor level into the air space between the open mold halves. To try to overcome this, advanced white-room optical or medical molding processes now position fans and blowers into enclosures mounted directly over the mold platens and tie bars, with a steady stream of air blown downward through HEPA filters of specified particle-size distribution. This forced air blast of relatively clean air tries to overcome the upwelling thermal currents of relatively dirty air and hopefully minimize exposure of the mold cavity's part-forming surfaces or the freshly molded part during its part-removal cycle from exposure to such airborne soils and contaminants.
This is an imperfect solution. It is nearly impossible to consistently maintain a dependable laminar air flow of suitable velocity to meet Class 100 clean-room standards in the immediate vicinity of the heated mold surfaces during their opening and closing sequence motions. Looking now at airborne particles of various size distribution, one is reminded that, although gravity is acting upon all the particles, the instantaneous velocities and direction of all particles in motion is not equal. Extremely fine, sub-micron-sized particles exhibit to a very great extent the random patterns of Brownian motion and have very slow overall settling rates or downward velocity. On the other hand, the largest particles, including those sufficiently large to be seen with the unaided human eye (typically, 20-50 microns in size) are acted on proportionately greater by gravity's forces overcoming the counteracting forces. Thus, the larger-sized airborne particles which are most flagrantly optical flaws, for example, are precisely those which have greater settling rates when the predominant air flow is directed downward, such as would be the case where air flow between the mold halves is predominantly from top-to-bottom.
Seeking to block such upwelling thermal air currents has been attempted by shrouding the mold on top and both sides with an accordion-like, fan-folded flexible material. Disadvantages are that it then blocks part-removal robot's access to the open mold, so the part must be dropped freely through the bottomside--which is unacceptable to delicate or scratch-sensitive surfaces of the molded part--and also the fact that the desired high-speed mold opening causes this apparatus to act like a bellows, sucking upward the dirtiest air.
Various conventional means exist for part-ejection systems and part-removal auxiliary robotics. For example, small aperatures in the mold can be made to either suck a vacuum or deliver blasts of compressed air behind the plastic part's interface with the mold surface, to thus aid, respectively, in firstly, the molded part to adhere onto the B mold surface, and secondly, in later stripping action and ejection of the part off of the B mold surface, when these bursts are properly coordinated into the mold-opening process sequence of ejection. Such apparatus is described in U.S. Pat. No. 4,573,900. A somewhat similar apparatus is described in U.S. Pat. No. 4,125,247 at column 5, lines 28-44. Related is the use of compressed gas as driving force to separate the molded part from the mold's partforming surfaces, where the gas enters at a selected point in the mold according to various designs and it is controllably started at the point in the press cycle when the mold is open for ejection. See U.S. Pat. Nos. 4,438,065 and 4,660,801.
Similarly, a variety of suction-cup or mechanical jaws for gripping molded parts are commonly placed onto the end of the arm of various robotic devices which enter the mold when its parting line is open and which attach themselves to the molded part, thus transferring it out of the mold according to a predetermined sequence, coordinated with the mold-opening sequence of the machine. See U.S. Pat. Nos. 4,368,018, 3,804,568, and 3,767,342.
A third way for controlled removal of the molded part is the use of collapsible cores which mechanically eliminate the undercut and corresponding retentive forces exerted by the plastic onto a certain predetermined portion of the mold. Since it is usually desirable to assure that the molded part is successfully transferred off of the stationary mold half (herein referred to as the "A" mold half), and is retained during the initial mold opening phases onto the movable mold half (herein referred to as the "B" mold half), controlled degrees of draft angle on the former and undercut or retention force on the latter is commonly employed. Once this first stage transfer on initial mold opening has been successfully accomplished, these controlled undercut or retention forces can be eliminated by collapsing mechanically these cores through a predetermined mechanism and sequence. An example of such is described in U.S. Pat. No. 4,627,810.
Once freed from the mold's partforming surfaces, the part may be assisted in its removal by gravity by various guide rails (U.S. Pat. Nos. 3,910,740 and 4,231,987) or discharge chutes. (U.S. Pat. No. 4,589,840).
However, the elimination of the basic problem of each of these elaborate and costly devices described above has not yet been solved. As long as the mold halves open at the parting line on each of the molding press cycles and thereby expose both the molded part and the mold's partforming surfaces to surrounding contaminated air, and as long as part-removal robotics is required to transfer the resulting molded part out of the open mold halves, costly clean room and robotic equipment will be required for contamination control.